The Magical Transformation Explained: How Our Trees Get Their Fall Colors

September is behind us, and the signs of fall are here. Pumpkins are appearing next to front doors, Halloween displays are up in stores and Edmonds' parks, streets and yards are showing the first blush of fall color.

But there's more to come. Beginning now and continuing through October, Edmonds' many native and non-native trees and shrubs will finish out the growing season in a brilliant display of fall colors, adding seasonal charm to our town.

But what happens to bring these colors out? How and why do these leaves go from summer greens to autumn reds, yellows and golds?

The answer lies in the pigments that are always present in leaves, primarily carotenes and xanthophylls—oranges and yellows. Chlorophyll, the  most important pigment, provides the green color.

During the growing season, chlorophyll dominates, masking the other pigments. Contained within the leaf cells in special bodies called chloroplasts, its job is to capture solar energy to run the plant's photosynthetic factory. Just as photovoltaic cells capture solar energy and convert it to electrical energy, chloroplasts capture solar energy and use it to drive the chemical reactions of photosynthesis, building sugars from the raw materials of water and carbon dioxide.

The cooler temperatures and shorter days of fall combine to tell plants that it's time to shut down the photosynthesis factory for the year. This triggers the annual process of abscission, sealing off the leaves from the stem and halting the flow of water to the leaf.

Without water, chlorophyll quickly breaks down and photosynthesis, the food-producing combination of sunlight, water, and carbon dioxide, stops.  Once the overwhelming chlorophyll color is removed, the yellows and oranges of the carotenes and xanthophylls begin to show through, and the annual fall color display begins.

This explains the oranges and yellows, but what about those spectacular reds, purples and scarlets?

These come from another class of pigments, the anthocyanins, which are not normally present during the growing season. Not all trees produce them, and even in those that do, the intensity varies with environmental factors such as daily temperature fluctuations and the presence or absence of early frost. The northeastern U.S., famed for its fall color displays, is blessed with the happy combination of scores of anthocyanin-producing trees and the environmental factors to trigger production of this pigment.

The Puget Sound area, while not known for the riotous fall color shows of the Northeast, still provides some striking seasonal displays. Our native vine and bigleaf maples (Acer circinatum and macrophyllum respectively) are the primary contributors. At higher elevations these are joined by manzanita (Arctostaphylos sp.), aspen (Populus sp.) and other broadleaf plants to produce fall color rivaling that of better-known regions.

In Edmonds, our city parks and private homes boast a wide array of non-native plants, many of which are chosen for their fall color. These include a variety of maples, oaks, sumacs, dogwoods, birches, hornbeams and barberries, all of which add to our local fall color. A great example are the pin oaks (Quercus palustris) surrounding the fountain at Fifth and Main. Native to the northeastern U.S., look for them to fill the fountain area with burnt umber and deep yellows over the next few weeks.

OK, I admit it. Autumn is my favorite season. I love the cool, crisp evenings, the warm days, and the fall colors. I hope knowing more about how these colors are produced helps deepen your appreciation of Edmonds' golden days of fall.

Larry Vogel, who also writes the regular history articles for Patch, holds undergraduate and graduate degrees in botany from the State University of New York, Syracuse University and the University of Washington. He has taught college-level biology, horticulture and environmental science, and has worked as a professional botanist for the Bureau of Land Managment.